My shower suddenly started leaking on Christmas Eve (have to keep turning cold water off at mains) - it is not the showerhead or hose but coming from little 'pipe' at bottom of shower unit itself(could it be the valve inside??). Would I be able to fix this myself (know nothing about plumbing but willing to try)or do you think it will require a plumber? (PS shower is about 6 yrs old)

The little pipe comes from the pressure relief valve. Once opened it cannot be shut, so you will need a new one from Triton. Call Triton and tell them the model and power rating of the shower. You can fit it yourself, but isolate the shower electrically before removing the cover.

Now then, the valve opens for a reason. Most often it is that the heat exchanger is scaled up. Remedy is the same as for the valve. Isolate before removing the cover.